You can technically do stacking in photoshop or any other image editor that supports layers, but a program like Nebulosity (available for Mac) is going to be a better bet. Bottom line is there is no iAstroPhoto product. Expect to spend some time reading tutorials and manuals. I use a Mac as well, and invested in a copy of Nebulosity early on. I don't regret it. You can download it as a trial copy that stamps the images, but is otherwise functional enough to give you a sense of the process. Craig Stark also has some really good tutorials on his site. http://stark-labs.com

I have not found any freeware stacking programs for Mac. At least none that have been updated in recent years.

*If* you have access to a copy of Windows running in VMWare Fusion or Parallels or BootCamp on your mac then get Deep Sky Stacker. I hate to give that advice but that is the way to do both easy and free stacking.

As already mentioned Nebulosity is a good choice for the inexpensive route. Not quite as simple as DSS is for stacking, but it serves well as Capture Control, Calibration & Stacking, and basic Post-stack Processing and is both Mac OS X and Windows compatible.